When a mobile radio receiver is switched on or when it enters a new cell, it must be synchronized to this cell or to potential cells. In order to associate the received data with the correct time, the data stream is organized in time flames and time slots. A frame contains a specific number of slots, and each slot contains a specific number of bits (data and/or control information). In particular, these cells must first of all be recognized and identified with respect to their timings (flame and slot start) and their base station codes. So-called synchronization sequences are generally used for this purpose, which have either global characteristics (which are the same throughout the network) or local characteristics (which differ for different base stations).
Synchronization of a third generation mobile radio receiver, such as UMTS, to a corresponding base station, that is to say time synchronization and identification of the cell code and scrambling code, is achieved by correlating the received signal with the complex-conjugate signal for the correspondingly transmitted synchronization code.
Since the synchronization sequences generally cannot be received without interference (AWGN), there is only a certain probability of the receiver system being synchronized to a corresponding cell. This synchronization probability depends essentially not only on the signal-to-noise ratio but also on the time interval available for the synchronization procedure. However, user-relevant and/or system-relevant aspects generally result in a link to specific timing requirements. If a synchronization process comprises N step elements (in the case of UMTS, three successive step elements are carried out: slot synchronization→frame synchronization+code group identification→scrambling code identification), then the detection probability after a step element n depends on that of the step element n−1 (n=2, . . . , N).
This results in two conclusions:                a) the step element n−1 should achieve a significantly higher detection probability than the step element n in order to result in an overall procedure with an efficient usefulness/complexity ratio,        b) detection errors/results in a step element should be recognized/evaluated in order to carry out the overall synchronization process with a predetermined detection probability in a minimum time.        
As has already been mentioned above, the synchronization procedure for a UMTS mobile radio receiver essentially comprises three step elements:                a) slot synchronization        b) flame synchronization and code group identification        c) scrambling code identification        
FIG. 2 illustrates a basic flowchart of one such conventional synchronization process for UMTS UEs.
Steps a) and b) in particular require a relatively long time since, in order to increase the detection probability, the received signal must be evaluated (averaged) over a total of two or more slots or even two or more frames. In order to further increase the detection reliability of a synchronization unit such as this, verification steps are normally inserted after one or more of the individual step elements, as is illustrated in FIG. 2. These verification steps must also be carried out over a relatively large number of slots or frames in order to obtain an appropriately reliable indication of the synchronization status of the receiver. This therefore means that additional time is required in order to improve the detection or synchronization probability.
As has already been mentioned above, the 3G system requirements result in specific time restrictions, so that only a specific number of slots or frames are available for a given synchronization task. The detection probability thus depends directly on this restriction. The minimum total detection time or total synchronization time is thus the sum of the slot synchronization time Tslot, of the slot boundary verification time Tveri-slot, of the frame synchronization time Tframe, of the frame verification time Tveri-frame and of the scrambling code identification time Tscr-id. On the assumption that the same time T is required for each step element, this results in a total time of Tmin-sync=5T.
Methods for synchronization with intermediate verification are also referred to as multi-dwell detectors. In this context, reference should be made to the book: Peterson, Ziemer, Borth: Introduction to Spread Spectrum Communication, page 255.